Because silicon nitride sintered products are excellent in various properties such as mechanical strength, heat resistance and corrosion resistance, etc., they are used as high temperature structural materials such as for parts of gas turbines. However, silicon nitride has a poor sinterability because of having a highly covalent bonding nature and, consequently, it is difficult to obtain silicon nitride having high density and high strength.
Hitherto, processes used for production of sintered products of silicon nitride have included (1) a reaction sintering process which comprises nitriding metallic silicon, (2) a pressureless sintering process and (3) a hot pressing process which comprises sintering a silicon nitride powder together with sintering aids. However, these processes have advantages and disadvantages shown in the following Table 1.
TABLE 1 ______________________________________ Process Advantage Disadvantage ______________________________________ (1) It is possible to Density is low, and Reaction sinter even in strength, corrosion sintering the case of a resistance and oxida- process product having a tion resistance are complicated shape. inferior. Degradation of strength hardly occurs even at high temperature. (2) It is possible to Deformation is easily Pressureless sinter even in caused by shrinkage sintering the case of a during sintering. process product having a The largest amount of complicated shape, sintering aids is but not as complex required, and conse- as in (1). quently degradation Strength is of strength is superior to that significant at high of (1). temperature. (3) Strength, oxida- It is only possible Hot pressing tion resistance to sinter products process and corrosion having a simple shape. resistance are Degradation of excellent. strength although lesser degree than (2) occurs at high temperature resulting from the addition of sintering aids. ______________________________________
All of these processes have advantages and disadvantages. Therefore, none of them are completely satisfactory processes for producing high temperature structural materials having complicated shapes, such as parts for gas turbines or engines, etc. However, since the pressureless sintering process and the hot pressing process each has certain advantages, they would become desirable processes for producing the high temperature structural materials, if a method of overcoming the disadvantages while keeping the advantages of each could be found out. Thus, various processes have been proposed hitherto. One of them is a process which comprises sintering a silicon nitride powder together with from 0.1 to 40% by weight of one or more components selected from titanium nitride and tantalum nitride. (Japanese Patent Application (OPI) No. 123113/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").) However, in this process, hot pressing which has the above mentioned disadvantages is necessary to obtain a fully dense products.